2. Field of the Invention
The present invention relates to the control of intercom line circuits used in key telephone systems in which exclusion circuits are provided in each key telephone set by using one control lead and two talking leads to generate signals indicative of and for the control over (1) the state of use of intercom lines, (2) seizure of idle intercome circuits, (3) response to intercom calls, and (4) access to an intercom circuit which is already being used by two telephone sets.
2. Description of the Prior Art
In prior art key telephone apparatus, the various controls over intercom circuits is performed with two talking leads, one status control lead and one lamp control lead, totalling four leads. Also, in the intercom circuits of certain prior art telephone apparatus, methods exist for excluding a third telephone set from an intercom line to which a connection exists between a calling telephone set and a called telephone set.
In the prior art key telephone apparatus described above, it has been necessary to have four leads for each intercom circuit, and as the number of intercom circuits accommodated by a key telephone set increases, the number of leads used increases accordingly.
In the prior art key telephone intercom line circuits, when one station attempted to call another station which was already engaged in conversation, control circuits common to all intercom line circuits were held busy until the calling station abandoned the attempt. This disrupted service to other users who might attempt to call other idle stations.
An object of the present invention is to provide apparatus for controlling intercom circuits in key telephone systems by using three leads instead of the four leads per intercom circuit as used in prior art key telephone apparatus.
A further object of this invention is to provide an exclusion circuit that prevents the telephone set of a third party from overhearing the calling and the called party and which is compatible with a three lead intercom control circuit in key telephone systems.
Still further objects are to reduce the number of relays required and minimize unnecessary use of common circuits which serve the plural intercom line circuits.